Electronic mail (email) communications have quickly become the primary communication vehicle for many corporations and enterprises due to its many advantages over non-electronic communications such as postal mail. These advantages include low cost, rapid delivery, ease of storage, and so on.
A corporation can deploy an edge server at the periphery of the corporate network, typically behind the corporate firewall, to secure their email communications. One function of the edge server is to receive all of the incoming email messages for the email clients configured in the corporation's email infrastructure, verify the email addresses, and relay the email messages to appropriate servers for delivery to the mailboxes. This allows the edge servers to verify the email addresses and to reject undesirable email messages, such as SPAM, before the messages are accepted into the corporate network. In order to perform this function, the edge server requires access to a complete and updated email recipient list, which is typically maintained on a directory server on the corporate network. Given the relatively high bandwidth provided by the corporate network (e.g., gigabyte Ethernet), the cost of periodically replicating the entire email recipient list from the directory server to the edge server whenever there is a change to the email recipient list is not significant. Even when the volume of email messages necessitates the deployment of multiple edge servers, periodic replication of the entire email recipient list from the directory server to each of the edge servers is still feasible as long as the size of the email recipient list is not prohibitively large.
As the volume of email messages increases and corporations become faced with the growing challenge of maintaining the integrity of their emails, corporations are increasingly turning to hosted email solutions. In some hosted email solutions, the hosted email service provider provides the edge servers within its hosted email service cloud (i.e., the collection of servers that provide the hosted email service functionality) replacing the edge servers that would previously have been deployed on premise. The edge servers deployed within the hosted email service cloud still require access to the complete and updated email recipient list that is provided on a directory server on the corporate network. Because the connectivity between the corporate network and the hosted email service cloud is typically via a much lower bandwidth connection, such as a T-1 connection, the cost associated with periodically replicating the entire email recipient list from the directory server on the corporate network to the edge server within the hosted email service cloud whenever there is a change to the email recipient list may become prohibitive.